The present invention generally relates to measurement devices. More specifically, the invention relates to interferometers for distance measurement.
In general, optical interferometry is the act of splitting and recombining electromagnetic waves, in particular, visible light waves, to measure surface geometries, distance, etc. The advancement in interferometry has come in many avenues of technology. Long-range telescopes, high-precision spectrometers, compact disc players, etc., use some form of interferometry. Micro-machinery is a growing technology field that often utilizes interferometers because, they typically have high resolution and precision. In general, displacement measurements in the sub-nanometer range can be detected with today""s interferometers. To examine microscale structures, the lateral resolution of the interferometers, generally, need to be improved. This can be achieved by coupling the interferometer to a regular microscope. Unfortunately, the size of the interferometer becomes rather large and subsequently may not fit in small spaces for inspection. Furthermore, to inspect a large number of microscale structures either the sample or microscope objective is scanned, resulting in slow imaging.
In order to obtain interferometric measurement sensitivity in a small volume, several methods have been developed. One of these methods involves phase sensitive diffraction gratings as described in a technical paper entitled xe2x80x9cInterdigital cantilevers for atomic force microscopy,xe2x80x9d published in Appl. Phys. Lett., 69, pp. 3944-6, Dec. 16, 1996 by S. R. Manalis, S. C. Minne, A. Atalar, and C. F Quate and also in U.S. Pat. No. 5,908,981 to Atalar et al.
Similar structures are also used in microaccelerometers to measure the displacement of a control mass with interferometric precision as described in a paper written by E. B. Cooper, E. R. Post, and S. Griffith and entitled xe2x80x9cHigh-resolution micromachined interferometric accelerometer,xe2x80x9d Appl. Phys. Lett., 76 (22), pp. 3316-3318, May 29, 2000. It should be noted, however, that these papers discuss measuring relative distance of the object with respect to the reference gratings.
Two well known uses for microinterferometers are range finding and shape measurement, of which there are several optical range finding and shape measurement methods. Traditional range finding using focus analysis is an effective method, but for high accuracy and reduced depth of field, the lenses are typically large. Hence, mechanical scanning to make shape measurement becomes a slow and difficult task. Microscopes can be used to enhance the resolution, but this comes at the cost of extremely short standoff distances from the object, making scanning difficult. Interferometric ranging methods are very accurate, but in ordinary implementations, the methods operate in a relative coordinate space and can be problematic when the object surfaces have abrupt discontinuities.
It would be desirable to have a microinterferometer that can determine an absolute distance, as opposed to most of today""s microinterferometers which can determine relative distance. It would also be desirable to increase the resolution and sensitivity of the microinterferometer, while keeping the microinterferometer relatively fast and relatively low in cost.
Based on the foregoing, it should be appreciated that there is a need for improved microinterferometers that address the aforementioned problems and/or other shortcomings of the prior art.
The present invention relates to microinterferometers. In this regard, a first embodiment of a microinterferometer for measuring the absolute distance to an object surface includes a substrate. The microinterferometer also includes a phase-sensitive, reflective diffraction grating formed on the substrate, the diffraction grating being configured to reflect a first portion of an incident light and transmit a second portion of the incident light, such that the second portion of the incident light is diffracted. The microinterferometer further includes a lens formed on the substrate for focusing the second portion of the incident light to a predetermined local distance, and a photo-detector for receiving interference patterns produced from the first portion of the incident light reflected from the diffraction grating and the second portion of the incident light reflected from the object surface.
A second embodiment of a microinterferometer in accordance with the present invention includes means for reflecting a first portion of an incident light and transmitting a second portion of the incident light, such that the second portion of the incident light is diffracted. The microinterferometer further includes means for focusing the second portion of the incident light on a predetermined local distance, and means for receiving interference patterns produced from the first portion of the incident light and the second portion of the incident light, wherein the second portion of the incident light has been reflected from an object surface.
A third embodiment of a microinterferometer in accordance with the present invention includes a substrate. The microinterferometer also includes at least a first diffracting micro-objective comprising the substrate, and a photo-detector for receiving interference patterns produced from a first portion of an incident light reflected from each diffracting micro-objective and a second portion of the incident light reflected from the object surface.
Embodiments of the invention may be construed as a diffracting micro-objective that includes a substrate. The diffracting micro-objective also includes a phase-sensitive, reflective diffraction grating formed on the substrate. The diffraction grating is configured to reflect a first portion of an incident light and transmit a second portion of the incident light, such that the second portion of the incident light is diffracted. The diffracting micro-objective also includes a lens formed on the substrate for focusing the second portion of the incident light to a predetermined local distance.
A representative method of fabricating a diffracting micro-objective is also provided. The method comprises the steps of: providing a substrate; forming a phase-sensitive, reflective diffraction grating on the substrate; and forming a microlens on the substrate.
A representative method for measuring the absolute distance to a target surface is also provided. The method comprises: illuminating the target surface with an incident light beam through a phase-sensitive, reflective diffraction grating, such that a first portion of the incident light beam is reflected and a second portion of the incident light beam is diffracted upon being transmitted through the diffraction grating; focusing the second portion of the incident light beam to a predetermined focal distance; receiving interference patterns produced from the first portion of he incident light beam reflected from the diffraction grating interfering with the second portion of the incident light beam reflected from the target surface; and measuring the intensity of the interference patterns to determine the absolute distance.
Other systems, methods, features, and advantages of the present invention will be or become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the present invention, and be protected by the accompanying claims.